1. Field of the Invention
This invention relates to hub clutches, and more particularly to an automatic free wheel hub clutch.
2. Description of the Related Art
An example of conventional automatic free wheel hub clutches (hereinafter referred to simply as "hub clutch") includes the one disclosed in U.S. Pat. No. 4,327,821 which comprises clutching splines and other splines formed on the outside circumference of a driving shaft and which are capable of engaging a clutch ring engaged slidably movably with splines on the inside circumference of a body, the clutch ring having splines capable of being in clutched engagement with said clutching splines adjacent the inside circumference, thereof a return spring stretchably provided between a clutch cap (covering member) and said clutch ring for urging the clutch ring towards the inside in the axial direction (clutched off direction), a cam follower disposed adjacent to the inside of the clutch ring in the axial direction and engaging movably along the axial direction with the other splines on the outside circumference of said driving shaft, said cam follower having a V-shaped projection projecting inwardly in the axial direction and a protrusion (tooth) formed on the outside circumference of the extreme end of said V-shaped projection and protruding towards the outside diameter direction, a cam member provided with a V-shaped grooved cam surface for mating with the V-shaped projection of said cam follower to seat it thereat on the outer circumference at the extreme end of the outside in the axial direction and secured to a stationary system (lock nut), a movable cam disposed coaxially on the outside diameter side of the cam member in a rotatable state and having a cam surface having a substantially V-shaped projection on the outside in the axial direction as well as a cam stop at the extreme end of said cam surface, and a drag shoe damping the movable cam with respect to the stationary system. A thrust washer is disposed at a sliding section defined between the inside surface of the drag shoe in the axial direction and the outside surface of the lock nut and which supports such urging force transmitted from the return spring through the clutch ring, the cam follower, the movable cam, and the drag shoe. On the inside diameter side of said return spring, a shift spring is coaxially disposed which is stretchably provided between the clutch cap and the clutching spline. Said clutch ring is in face to face contact with the cam follower in a slidable state.
In the above construction, when torque is transmitted from said other spline to the cam follower by rotating the driving shaft, the V-shaped projection of the cam follower which has been fitted into the V-shape grooved cam surface of the cam member shifts to the outside in the axial direction along said V-shaped grooved cam surface. When said V-shaped projection gets over said V-shaped groove, the projection at the extreme end engages, at this time, with the V-shaped protruding cam surface of the movable cam (rotation of which is suppressed by the drag shoe), and shifts towards the outside in the axial direction along the same. At the time when said projection gets over the V-shaped groove, the clutching spline on the outer circumference of the driving shaft is already in clutched engagement with the clutch ring, and the locking state thereof becomes deeper with the shift of the projection towards the outside in the axial direction along the projecting cam surface of the movable cam, and such a locking state finishes when the projection engages with a cam stop. The drag shoe controls the movable cam so that such a clutched engagement as described above can be maintained.
Furthermore all of said clutching splines and the other splines are disposed on the driving shaft adjacently to each other in an axially movable condition, and the displacement of these splines towards the outside in the axial direction is elastically suppressed by means of the shift spring provided stretchably between the clutching splines (positioned outwardly along the axial direction) and said clutch cap.
In conventional hub clutches, however, when a clutch is engaged (in case of locking condition), the urging force from a return spring is received by a thrust washer through a clutch ring, a cam follower, a movable cam, a drag shoe and the like. Hence the spring load in this case has no function and it produces a wasted force. Furthermore wear or durability in an engaged portion where load is received thereby are affected, so that there is a fear of an accompanying decrease in performance. Besides, the drag shoe functions to maintain the clutched engagement condition (one before a projection of the cam follower engages with a cam stop after having engaged with a V-shape projecting cam of the movable cam), while such a function itself of commencing the clutched on-off operation is performed by a cam member which is a stationary system. In other words, the function for starting the clutched on-off operation and the function for maintaining the clutched on condition are shared by separate members so that the members used are increased and the construction therefor becomes complicated.
In addition, since a cam mechanism producing a thrust force required for clutched engagement is comprised of cam surfaces sliding with each other, the brake section generating damping force for the cams may require a high damping resistance so that there is a possibility of a decrease in decreasing durability. On one hand, if the damping resistance is insufficient, the clutching operation becomes unreliable.
Moreover, in conventional hub clutches, since the inside end surface of said clutch ring in the axial direction contacts slidably with the outside end surface of the cam follower, there is a possibility of wearing the sliding surface of said clutch ring and said cam follower (sliding due to relative rotation of both the members) by means of the load from the return spring and the shift spring in the case of ratcheting of the clutching spline and the clutch ring (such a state where both the end surfaces merely contact with each other and no meshing is observed) in the course of shifting a locked state, and as a result there is also a possibility of a reduction in durability. More specifically, the clutching spline is urged by the cam follower towards the outside along the axial direction at the time of ratcheting, so that the clutching spline is displaced outwardly in the axial direction to compress the shift spring. Thus, in addition to the urging force from the return spring, another urging force, i.e. a reaction force of the shift spring, is applied to the contacting surface (sliding surface) of the clutch ring and the cam follower, so that wear of said sliding surface is accelerated by a synergistic effect derived from both the urging forces and the durability thereof decreases remarkably.